The importance of PUFAs is undisputed. For example, certain PUFAs are important biological components of healthy cells and are recognized as: “essential” fatty acids that cannot be synthesized de novo in mammals and instead must be obtained either in the diet or derived by further desaturation and elongation of linoleic acid (LA; 18:2 ω-6) or α-linolenic acid (ALA; 18:3 ω-3); constituents of plasma membranes of cells, where they may be found in such forms as phospholipids or triacylglycerols; necessary for proper development (particularly in the developing infant brain) and for tissue formation and repair; and, precursors to several biologically active eicosanoids of importance in mammals (e.g., prostacyclins, eicosanoids, leukotrienes, prostaglandins). Additionally, a high intake of long-chain ω-3 PUFAs produces cardiovascular protective effects (Dyerberg, J. et al., Amer. J. Clin. Nutr., 28:958-966 (1975); Dyerberg, J. et al., Lancet, 2(8081):117-119 (Jul. 15, 1978); Shimokawa, H., World Rev. Nutr. Diet, 88:100-108 (2001); von Schacky, C. and Dyerberg, J., World Rev. Nutr. Diet, 88:90-99 (2001)). And, numerous other studies document wide-ranging health benefits conferred by administration of ω-3 and/or ω-6 PUFAs against a variety of symptoms and diseases (e.g., asthma, psoriasis, eczema, diabetes, cancer).
A variety of different hosts including plants, algae, fungi and yeast are being investigated as means for commercial PUFA production. Genetic engineering has demonstrated that the natural abilities of some hosts (even those natively limited to LA and ALA fatty acid production) can be substantially altered to result in high-level production of various long-chain ω-3/ω-6 PUFAs. Whether this is the result of natural abilities or recombinant technology, production of arachidonic acid (ARA; 20:4 ω-6), eicosapentaenoic acid (EPA; 20:5 ω-3) and docosahexaenoic acid (DHA; 22:6 ω-3) may all require expression of a Δ5 desaturase.
Most Δ5 desaturase enzymes identified so far have the primary ability to convert dihomo-γ-linolenic acid (DGLA; 20:3 ω-6) to ARA, with secondary activity in converting eicosatetraenoic acid (ETA; 20:4 ω-3) to EPA (where DHA is subsequently synthesized from EPA following reaction with an additional C20/22 elongase and a Δ4 desaturase). The Δ5 desaturase has a role in both the Δ6 desaturase/Δ6 elongase pathway (which is predominantly found in algae, mosses, fungi, nematodes and humans and which is characterized by the production of γ-linoleic acid (GLA; 18:3 ω-6) and/or stearidonic acid (STA; 18:4 ω-3)) and the Δ9 elongase/Δ8 desaturase pathway (which operates in some organisms, such as euglenoid species and which is characterized by the production of eicosadienoic acid (EDA; 20:2 ω-6) and/or eicosatrienoic acid (ETrA; 20:3 ω-3)) (FIG. 1).
Based on the role Δ5 desaturase enzymes play in the synthesis of e.g., ARA, EPA and DHA, there has been considerable effort to identify and characterize these enzymes from various sources. As such, numerous Δ5 desaturases have been disclosed in both the open literature (e.g., GenBank Accession No. AF199596, No. AF226273, No. AF320509, No. AB072976, No. AF489588, No. AJ510244, No. AF419297, No. AF07879, No. AF067654 and No. AB022097) and the patent literature (e.g., U.S. Pat. Nos. 5,972,664 and 6,075,183; see also commonly owned, co-pending Provisional Applications No. 60/801,119 (filed May 17, 2006) disclosing amino acid and nucleic acid sequences for a Δ5 desaturase enzyme from Peridium sp. CCMP626; and No. 60/915,733 (filed May 3, 2007) disclosing amino acid and nucleic acid sequences for a □5 desaturase enzyme from Euglena anabaena. 
Applicants have identified and isolated new genes encoding □5 desaturases that are be suitable for heterologous expression in a variety of host organisms for use in the production of ω-3/ω-6 fatty acids. Specifically new genes encoding Δ5 desaturase from Euglena gracilis have been discovered.